Journal Club: What’s old is new again: newly discovered songbird family is ancient

SUMMARY: Scientists analysing songbird DNA discovered that the spotted wren-babbler is neither a wren nor a wren-babbler, nor even a babbler. Instead, it represents an old evolutionary family that has no close living relatives.

A newly published study has uncovered a previously unknown family of songbirds that is represented by just one species. After analyzing one of the largest and most comprehensive songbird DNA databases amassed so far, researchers recovered ten distinct avian family branches in the Passerida songbird family tree, including the newly identified family, represented by the spotted wren-babbler, which lives in Asia. The data reveal that the spotted wren-babbler is neither a wren nor a wren-babbler and in fact, it has no close living relatives at all. The researchers concluded that the spotted wren-babbler is the sole representative of a unique avian family that is the earliest surviving evolutionary offshoot in Passerida.

We know more about birds than we know about any other animal group on Earth. Even on the remotest places on the planet, someone seems to be studying, filming, photographing, feeding, hunting, or watching birds. So you might think that birds have no big secrets left for us to discover.

If you think this, then you would be wrong.

A newly-published study has uncovered a surprise: a previously unknown family of songbirds that is represented by just one species. This songbird is the spotted wren-babbler, Elachura formosa (formerly Spelaeornis formosus), a little brown bird that is so shy that it's heard more often than seen as it goes about its business in dense moist forests throughout the mountains of tropical and subtropical Asia (figure 1, larger view):

This discovery was reported by an international team of researchers led by Per Alström, Associate Professor at the Swedish University of Agricultural Sciences in Uppsala and Visiting Professor at the Chinese Academy of Sciences in Beijing. This group of scientists are working to clarify our understanding of the evolutionary history of Asian songbirds, particularly those that reside in the Himalaya Mountains.

Since DNA technology is becoming more accessible and computers are getting faster at analysing the large DNA datasets used to reconstruct phylogenies (family trees), our understanding of the evolutionary relationships amongst birds has repeatedly -- and sometimes dramatically -- changed during the past ten years. Many of these changes have been within the (mostly) Old World group of songbirds that are collectively known as "babblers".

Building upon previous molecular research, which recognises five lineages, or "clades", of babblers -- Timaliidae (Old World babblers), Leiotrichidae, Pellorneidae (ground babblers), Zosteropidae (white-eyes) and Sylviidae (Old World Warblers) -- the team collected DNA samples from representatives of all known families within the large songbird radiation, Passerida, which encompasses the five major lineages that they were targeting.

The team also included samples from some birds, particularly the Spelaeornis species -- the so-called "wren-babblers" -- that have baffled ornithologists for decades. For example, only in 2007 was it realised that Spelaeornis comprised eight species instead of five (Collar & Robson). Further, one of those newly-recognised species, the spotted wren-babbler, Spelaeornis formosus, had sufficiently distinctive morphology and vocalisations to remove it to a monotypic genus, Elachura.

But did the team expect more big surprises from this tiny chocolate-brown bird?

"The first [spotted wren-babbler] sample that we sequenced, from northeastern India ... appeared in a totally unexpected position, [so] we decided to collect more samples", said Professor Alström in email. Based upon this unanticipated finding, Professor Alström's team had a hunch that they might find something interesting if they investigated more thoroughly.

The team assembled one of the largest DNA sequence datasets ever amassed for the Passerida group, both in number of species and loci represented, and analysed those data to reconstruct a multilocus family tree (figure 2; larger view):

Figure 2. Relationships of the primary lineages within Passerida based on mitochondrial cytb and ND2 and nuclear myoglobin, ODC, GAPDH, LDH and RAG-1, analysed by Bayesian inference in 10 partitions under a relaxed clock model. Posterior probabilities (PP) and maximum-likelihood bootstrap (MLBS) values are shown at the nodes, in this order; asterisk (*) indicates PP 1.00 or MLBS 100% [doi:10.1098/rsbl.2013.1067]

But as you can clearly see, the hero of our story, the cute little spotted wren-babbler, Spelaeornis formosus, perches alone on a thin, bare twig amongst all these big basal branches (see the blue box on the far right, or refer to the closer look below, where that very special blue box appears near the bottom on the right side of the image):

Now that was a surprise. These data reveal that the spotted wren-babbler apparently typifies an old lineage within this larger passerine family tree -- a lone species representing a relictual avian family with no close living relatives.

"Within Passerida, which contains [approximately] 36 percent of all birds and 60 percent of all passerines, the spotted wren-babbler is unique, as it is the only extant species that on its own represents one of the most basal lineages", write the authors in their paper, which was just published in the peer-reviewed journal, Biology Letters.

"I doubt that we'll find another species that belongs in the same family. So the Elachura [spotted wren-babbler] is likely to be a true loner", said Professor Alström in email.

So how did this one tiny songbird manage to fool so many taxonomic experts and ornithologists for more than 100 years? Thanks to the wonders of convergent evolution, unrelated species evolve to resemble each other because they occupy similar niches. (Think: sharks and dolphins.) When convergent evolution occurs within a group of rapidly radiating animals, such as songbirds, it becomes very difficult, or just plain impossible, to determine which characters are most important for making taxonomic classifications.

Has Professor Alström ever seen this bird alive?

"Yes, I've seen it several times, both in India and China. It's extremely difficult to watch, as it inhabits dense undergrowth, often in little gullies on very steep mountain slopes", replied Professor Alström in email.

"All of the ones that I've seen I've first noticed by their distinctive song."

In their paper, the researchers proposed the new family name, Elachuridae, and recommended that the scientific name Elachura formosa be officially adopted. They also suggested that the bird's English common name be changed to Elachura. Yet despite all these big changes, this diminutive bird still retains its spots.